1. Field of The Invention
This invention relates to a system and method for producing a pilot signal in a distortion reduction system, for example in a signal amplification system.
2. Description of Related Art
Amplifiers often add undesired distortion to a signal, creating an output signal comprising distortion or nonlinear components and the signal component. The distortion includes any undesired signals added to or affecting adversely the input signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modern amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of input RF carrier pattern as well as the resultant distortion location, a known frequency component, i.e. a pilot signal, is injected in the main signal path with the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot signal along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot signal, the distortion can also be removed.
The pilot signal is an electrical signal comprising at least one frequency component spectrally located near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in FIG. 1 which shows the frequency response of a radio frequency (RF) amplifier including the location of the pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot 1) and/or located near the upper edge of the band of operation (e.g., pilot 2). The pilot is positioned a spectral distance of xcex94ƒ from an edge of the band of operation whose center frequency is ƒ0. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known.
The feed forward distortion reduction circuitry reduces distortion produced by the RF amplifier by applying the pilot signal to the RF amplifier and making adjustments based on information obtained from the applied pilot signal. FIG. 2 discloses feed-forward correction circuitry 10 and its use of information obtained from the pilot signal to reduce distortion produced by RF amplifier 12. An input signal, for example including at least one carrier signal, is applied to a splitter 14. The splitter 14 replicates the input signal on a main signal path 16 and a feed forward path 18. The splitter 14 is part of a carrier cancellation loop referred to as loop # 1, which in addition to the splitter 14, comprises gain and phase circuit 20, coupler 22, the RF amplifier 12, delay circuit 24 and couplers 26 and 28. The signal on the main path 16 is applied to gain and phase circuit 20. The output of gain and phase circuit 20 and the pilot signal are applied to the coupler 22. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of the amplifier 12. The output of the coupler 22 is applied to the amplifier 12 whose output comprises the amplified input signal, the amplified pilot signal and distortion, signals produced by the amplifier 12.
A portion of the output of the amplifier 12 is obtained from the coupler 26 and is combined at the coupler 28 via coupling path 30 with a delayed version of the input signal on the feed forward path 18 to isolate the pilot signal with distortion on the feed forward path 18. The input signal on the feed forward path 18 is sufficiently delayed by delay circuit 24 so that such signal experiences the same delay as the signal appearing at the coupler 28 via the path 30. The resulting error signal contains the distortion produced by the amplifier 12 along with any portion of the carrier signal remaining at the output of the coupler 28 and the pilot signal. The amount of carrier cancellation in the carrier cancellation loop depends on the proper gain and out-of-phase relationship between the two paths from the splitter 14 to the coupler 28.
The gain and phase circuit 20 adjusts the phase and gain of the input signal according to control signals on control paths 32 and 33 such that the signal appearing at the coupler 28 via the path 30 is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the delayed input signal at the coupler 28. The gain and phase control signals appearing on the control paths 32 and 33 of the gain and phase circuit 20 are derived from the signal at the output of the coupler 28 in a well known manner by sampling the output of the coupler 28 with a coupler 34 and using signal detection and control circuitry 35. In general, the signal detection and control circuitry 35 detects an error signal for the carrier:cancellation loop. The error signal represents the amplitude of the carrier signal(s) at point A, and the signal detection and control circuitry 35 attempts to reduce the amplitude of the carrier signal(s) at point A by providing gain and/or phase control signals.
In this embodiment, the signal detection and control circuitry 35 includes a detector 36, such as a log detector, which produces a signal representing the amplitude of the carrier signal(s) at point A. A filter 38 filters the output of the log detector to produce a DC-type amplitude signal representing the amplitude of the carrier signal(s). The amplitude signal is provided to a nulling circuit 40. In response to the amplitude signal, the nulling circuit 40 provides the control signals on the control paths 32 and 33 to adjust the relative gain and/or phase between the combining signals at the coupler 28 and reduce the carrier signal(s). When the carrier signal(s) is minimized, the carrier signals combined at the coupler 28 substantially cancel each other leaving at the output of the coupler 28 the pilot signal with distortion produced by the amplifier 12. Loop # 1 is thus a carrier cancellation loop which serves to isolate on the feed forward path 18 the pilot signal with distortion produced by the amplifier 12.
A distortion reduction loop or loop # 2 attempts to reduce the pilot signal on the main signal path 16, thereby reducing the distortion produced by the amplifier 12, using the output of the coupler 28. The pilot signal with distortion on the feed forward path 18 is fed to a gain and phase circuit 42. The output of the gain and phase circuit 42 is fed to amplifier 44 whose output is applied to coupler 46. The coupler 46 combines the amplified pilot signal and distortion on the feed forward path 18 with the signals from the amplifier 12 on the main signal path 16 (carrier signal(s), pilot signal with distortion). A delay circuit 40 on the main signal path 16 delays the signals from the output of the amplifier 12 on the main signal path 16 to experience substantially the same delay as the corresponding signals from the output of the amplifier 12 which pass over the coupling path 30 through the coupler 28 to the coupler 46.
A coupler 48 provides an error signal representative of the signal at the output of the coupler 46 onto a pilot detection path 50. Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detection and control circuitry 52 can detect the amplitude of the remaining portion of the pilot signal from the error signal on the pilot detection path 50. The pilot detection and control circuitry 52 determines the amplitude of the pilot signal, and in response to the amplitude of the remaining pilot signal, the pilot detection and control circuitry 52 provides control signals to the phase and gain circuit 42. In general, the pilot detection and control circuitry 52 will detect the pilot signal and use this information to generate control signals onto paths 66 and 68 to cause the gain and phase circuit 42 to adjust the gain and/or phase of the pilot signal on the feed forward path 18 such that the pilot signal on the main path 16 as well as the distortion is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the pilot signal and the distortion on the feed forward path 18 at the coupler 46. The corresponding pilot signals and distortion substantially cancel each other at the coupler 46 leaving the carrier signal(s) at the output of the system. Therefore, loop # 2 is a distortion reduction loop which attempts to cancel the pilot signal to cancel substantially the distortion produced by the amplifier 12.
In this embodiment, the pilot detection and control circuitry 52 includes pilot receive circuitry 54 which includes a mixer 56 to frequency convert the error signal on the pilot detection path 50 to lower frequencies and a filter 58 to facilitate detection of the pilot signal by a signal detector 60. The detector 60, such as a log detector, produces a signal representing the amplitude of the signal at the output of the coupler 46. A filter 62 filters the output of the detector 60 to produce a DC-type amplitude signal representing the amplitude of the remaining pilot signal. The amplitude signal is provided to a nulling circuit 64. In response to the amplitude signal, the nulling circuit 64 provides gain and phase control signals on the control paths 66 and 68 to the phase and gain circuit 42. The control signals are provided to adjust the relative gain and/or phase between the pilot signals being combined at the coupler 46 and reduce the remaining pilot signal. The amount of cancellation of the pilot signal indicates the amount of distortion cancellation. When amplitude of the pilot signal is minimized, the pilot signals and distortion combined at the coupler 46 substantially cancel each other at the output of the coupler 46.
The distortion of the input signal causes power to be generated in adjacent channels or frequencies to corrupt or interfere with signals in the adjacent channels or frequencies, commonly referred to as spectral regrowth or adjacent channel power (ACP). The generation of adjacent channel power is of particular concern in wireless communications systems where adjacent channel power of one channel interferes with other channels or frequency bands. Wireless cellular communications systems comprise a number of base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units, which can be mobile or fixed, in the geographic region. Each base station handles voice and/or data communications over a particular region called a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system""s coverage area.
In a wireless cellular communications system, a base station and a wireless unit communicate voice and/or data over a forward link and a reverse link, wherein the forward link carries communication signals from the base station to the wireless unit and the reverse link carries communication signals from the wireless unit to the base station. There are many different schemes for determining how wireless units and base stations communicate in a cellular communications system. Multi-user wireless communications systems, such as Code division multiple access (CDMA), wideband CDMA, Time division multiple access (TDMA), Global System for Mobile Communications (GSM) and orthogonal frequency division multiplexing (OFDM), multiple voice and/or traffic channels are combined into a single or multiple carriers. A linear amplifier should be able to react rapidly to transmit power changes and bursty traffic variations within the transient response specifications in the microsecond and millisecond ranges while providing adequate error cancellation.
In actual systems, there is rarely an absolute cancellation of the distortion and the pilot signals. Feed forward distortion reduction systems require tight operating tolerances, for example to achieve a 30 dB reduction in IMDs, typical feed forward correction system may have a frequency flat response (amplitude deviation over the frequency band of operation) as low as +or xe2x88x920.1 dB and phase linearity (phase deviation from a straight line in the frequency band of operation) as low as +or xe2x88x921 degree. To obtain this accuracy is difficult. In feed forward distortion reduction systems which use a pilot signal, the amplitude of the pilot signal is typically relatively small at the output of the feed forward distortion reduction system. Accordingly, if the pilot signal is positioned too close to a carrier signal, the pilot signal can be difficult to detect because of the relatively small amplitude of the pilot signal with respect to the amplitude of the carrier signal. Thus, it becomes difficult to detect the pilot signal at the output of the system. If the pilot signal is positioned away from the carrier signal(s) and the resulting IMDs, the pilot signal may be easier to detect but the cancellation of the distortion could suffer because of nonlinearities in the frequency response or phase response of the amplifier over the entire frequency band of operation. To improve detection of the pilot signal at the output of the distortion reduction system, schemes have been developed to scan the in-band frequencies and position the pilot signal at a xe2x80x9cquietxe2x80x9d spot in the frequency band of operation which is as close as possible to the active carriers. Such schemes may not react to changing numbers of transmit frequencies.
The present invention involves a distortion reduction system using upstream signal information, such as the carrier frequencies in an input signal, to adjust at least one frequency for a pilot signal to be injected into the distortion reduction system and to be detected at the output of the distortion reduction system, thereby enabling improved distortion reduction of changing input signals. For example, processing circuitry obtains the traffic frequencies making up a signal to be amplified by a feed forward arrangement. Using the traffic frequencies, the processing circuitry determines at least one frequency for a pilot signal, and the processing circuitry tunes a pilot signal generator to the at least one frequency for the pilot signal. The feed forward arrangement receives the signal to be amplified and provides replicas of the signal on a main signal path and on a feed forward path. The pilot signal is injected into the main signal path at the at least one frequency along with the signal to be amplified. The signal and the pilot signal are amplified by an amplifier on the main signal path, resulting in the amplified signal, the amplified pilot signal and distortion signals produced by the amplifier. A replica of the output of the amplifier is placed on a coupling path and combined with a delayed version of the signal on the feed forward path to isolate a replica of the pilot signal and distortion on the feed forward path. The pilot signal and distortion on the feed forward path are combined with the amplified signal, the amplified pilot and the distortion on the main signal path. A sample of the output is provided to pilot detection circuitry which is tuned by the processing circuitry to detect the pilot signal at the at least one frequency. Depending on the amplitude of the pilot signal at the output, the processing circuitry provides gain and/or phase control signal(s) to adjust the relative phase and/or gain between the combining pilot signals to reduce the amplitude of the pilot signal at the output and thereby to reduce the amplitude of the distortion.